Recently, a class of cell surface proteins have been described in both plants and animals that are involved in pathogen perception, MHC class II trans-activation , inflammation and the regulation of apoptosis (Inhere, N., Nunez, G, Cell, Death, Differ., 6(9):823–4, (1999); Inhere, N., Koneke, T., del, Peso, L., Hun, Y., Yee, C., Chen, S., Carrion, R., Merino, J., Liu, D., Ni, J., Nunez, G, J. Biol, Chem. 21., 274(21):14560–7, (1999); Inhere, N., Nunez, G, Cell, Death, Differ., 7(5):509–10, (2000); Harton, J A., Ting, J P, Mol, Cell, Biol., 20(17):6185–94, (2000); Dixon, J., Brakebusch, C., Fassler, R., Dixon, M J. Hum, Mol, Genet. 12., 9(10):1473–80, (2000)). All of these proteins are modular in nature containing one or several domains that function in caspase recruitment (CARD), nucleotide binding and protein-protein interactions. Proteins within this group have also been found to play a role in cell adhesion during various developmental processes.
A common theme in all of these proteins are the presence of a leucine-repeat repeat (LLR) in the carboxy terminus of the polypeptide chain. LLRs are short protein modules characterized by a periodic distribution of hydrophobic amino acids, especially leucine residues separated by hydrophilic residues [Sean, 1996]. The basic structure of the repeat is as follows:                X-L-X-X-L-X-L-X-X-N-X-a-X-X-X-a-X-X-L-X_SEQ ID NO:34)where X is any amino acid, L is leucine, N is asparagine and “a” denotes an aliphatic residue. The asparagine at position 10 can be replaced by cysteine, threonine or glutamine. The average repeat length is 24 amino acids but it can vary between 22 to 29 amino acids, though some LRR motifs have been reported to be at short as 20 amino acids. The motif often consists of leucine or other aliphatic residues at positions 2, 5, 7, 12, 16, 21, and 24 and asparagine, cysteine or threonine at position 10. X-ray structure determination of LRR motifs suggests that each LRR is composed of a beta-sheet and an alpha-helix. The largest subfamily of proteins that contain a leucine-rich domain are extracellular proteins having the following motif: LxxLxxLxLxxNxLxxLPxxOFxx (SEQ ID NO:35), where “x” is any amino acid and “O” is a non-polar residue (Kajava, J. Mol. Biol. 277: 519 (1998)).        
In transmembrane proteins, LLRs and their flanking sequence always occur in the presumed extracellular portions. In these situations the LLRs are generally flanked on either side by cysteine-rich regions. In general, these cysteines are present in the oxidized disulphide link form. An example of a transmembrane protein containing a LRR is Toll, a Drosophila gene the functions in establishment of dorsal-ventral patterning. Dominant, ventralizing mutants have been described that map to the cysteine-rich regions surrounding the LLR domain [Schneider, 1991]. Thus, the cysteine regions associated with LLRs act to regulate receptor activity. The LLRs themselves within the Toll protein have been shown to function in heterotypic cell adhesion, a process required for proper motorneuron and muscle development [Halfon, 1995]
Another Drosophila LLR containing transmembrane protein, 18 wheeler, which is regulated by homeotic genes also promotes heterophilic cell adhesion in cell migration events during development (Eldon, E., Kooyer, S., D'Evelyn, D., Duman, M., Lawinger, P., Botas, J., Bellen, H, Development., 120(4):885–99, (1994)). Mammalian CD14, which binds lipopolysaccharide (LPS), and signals through NF-κB, is thought to have analogies to the Toll signal transduction pathway. CD14 also contains a region of LLRs that have been shown in deletion mutants to be responsible for LPS binding.
Slit is another LLR containing Drosophila secreted protein that functions in the development of the midline glial cells and the commissural axon tracts the cross the midline. This is presumably accomplished by cell adhesion events (Jacobs, J R, J. Neurobiol., 24(5):611–26, (1993)). Mammalian homologues of Drosophila slit have been shown to bind the heparin sulfate proteoglycan, glypican-1 (Liang, Y., Annan, R S., Carr, S A., Popp, S., Mevissen, M., olis, R K., olis, R U, J. Biol, Chem. 18., 274(25):17885–92, (1999)). In general, heparan sulfate proteoglycans have been shown to accummulate in Alzheimer's disease brains and specifically, glypican-1 is component of both senile plaques and neurofibrillary tangles (Verbeek, M M., Otte, Holler, I., van, den, Born, J., van, den, Heuvel, L P., David, G., Wesseling, P., de, Waal, R M, Am. J. Pathol., 155(6):2115–25, (1999)). Heparin sulfate proteoglycans are also implicated in the regulation of cytokine signaling in B cells through the activation of CD40 (van, der, Voort, R., Taher, T E., Derksen, P W., Spaargaren, M., van, der, Neut, R., Pals, S T, Adv, Cancer, Res., 79:39–90, (2000)).
p37NB is a 37 kDa LRR protein identified in human neuroblastoma cells (Kim, D. et al. (1996) Biochim. Biophys. Acta 1309: 183–188). Northern blot hybridization and RT-PCR studies show that p37NB is differentially expressed in several neuroblastoma cell lines. A related LRR protein, PRELP, is characterized as a 42 kDa secreted protein (Bengtsson, E. et al. (1995) J. Biol. Chem. 270: 25639–25644). PRELP consists of 10 LRR motifs ranging in length from 20 to 26 residues with asparigine at position 10. Northern analysis shows differential expression of PRELP in various tissues.
In addition, leucine-rich repeat containing proteins have also been implicated in various aspects of protein-protein interaction, such as cell-to-cell communication and signal transduction (for a review, see Kobe and Deisenhofer, TIBS 19: 415 (1994); Kobe and Deisenhofer, Curr. Opin. Struct. Biol. 5: 409 (1995); Kajava, J. Mol. Biol. 277: 519 (1998)). Proteins that contain an LRR motif include hormone receptors, enzyme subunits, cell adhesion proteins, and ribosome-binding proteins.
A subfamily of the LRR superfamily, referred to as the Small Leucine Rich Proteoglycan family, illustrates the critical functions fulfilled by proteins containing an LRR motif. Members of this subfamily are believed to play essential biological roles during inflammation and cancer invasion, a regulatory role in collagen fibril formation, suppression of the malignant phenotype of cancer cells, and an inhibition of the growth of certain normal cells (see, for example, Iozzo, Annu. Rev Biochem. 67: 609 (1998)).
Kajava, et al., J. Mol. Biol. 277: 519 (1998), divided the LRR superfamily into subfamilies characterized by different lengths and consensus sequences of the leucine-rich repeats. Based upon this structural analysis, Kajava concluded that LRR proteins of different subfamilies probably emerged independently during evolution, indicating that proteins with the LRR motif provide a unique solution for a wide range of biological functions.
LLR containing proteins have been identified in prokaryotes, plants, yeast and mammals. Although such proteins were initially thought to be secreted proteins, it is now appreciated that they inhibit a variety of cellular locations and participate in a diverse set of critical functions in development and cellular homeostasis.
Such LRRs, being extracellular, are capable of directing protein-protein interactions with other receptors involved in apoptosis, inflammation and immune responses. LLR containing proteins may also bind other extracellular ligands derived from infectious agents and participate in the triggering and or modulating immune responses, particularly apoptosis.
The mechanisms that mediate apoptosis have been intensively studied. These mechanisms involve the activation of endogenous proteases, loss of mitochondrial function, and structural changes such as disruption of the cytoskeleton, cell shrinkage, membrane blebbing, and nuclear condensation due to degradation of DNA.
The various signals that trigger apoptosis are thought to bring about these events by converging on a common cell death pathway, the core components of which are highly conserved from worms, such as C. elegans, to humans. In fact, invertebrate model systems have been invaluable tools in identifying and characterizing the genes that control apoptosis. Despite this conservation of certain core components, apoptotic signaling in mammals is much more complex than in invertebrates. For example, in mammals there are multiple homologues of the core components in the cell death signaling pathway.
Caspases, a class of proteins central to the apoptotic program, are responsible for the degradation of cellular proteins that leads to the morphological changes seen in cells undergoing apoptosis. Caspases (cysteinyl aspartate-specific proteinases) are cysteine proteases having specificity for aspartate at the substrate cleavage site. Generally, caspases are classified as either initiator caspases or effector caspases, both of which are zymogens that are activated by proteolysis that generates an active species. An effector caspase is activated by an initiator caspase which cleaves the effector caspase.
Initiator caspases are activated by an autoproteolytic mechanism that is often dependent upon oligomerization directed by association of the caspase with an adapter molecule.
Apoptotic signaling is dependent on protein-protein interactions. At least three different protein-protein interaction domains, the death domain, the death effector domain and the caspase recruitment domain (CARD), have been identified within proteins involved in apoptosis. A fourth protein-protein interaction domain, the death recruiting domain (DRD) was recently identified in murine FLASH (Imai et al. (1999) Nature 398: 777–85).
Caspases comprise a multi-gene family having at least 12 distinct family members (Nicholson (1999) Cell Death and Differentiation 6: 1028). A relatively small fraction of cellular polypeptides (less than 200) are thought to serve as targets for cleavage by caspases. Because many of these caspase targets perform key cellular functions, their proteolysis is thought to account for the cellular and morphological events that occur during apoptosis. Members of the caspase gene family can be divided by phylogenetic analysis into two major subfamilies, based upon their relatedness to ICE (interleukin-lp converting enzyme; caspase-1) and CED-3. Alternate groupings of caspases can be made based upon their substrate specificities. Many caspases and proteins that interact with caspases possess a CARD domain.
Characterization of the HLRRBM1 polypeptide of the present invention led to the determination that it is involved in the modulation of the p21 G1/G2 cell cycle check point protein, either directly or indirectly.
Critical transitions through the cell cycle are highly regulated by distinct protein kinase complexes, each composed of a cyclin regulatory and a cyclin-dependent kinase (cdk) catalytic subunit (for review see Draetta, 1994). These proteins regulate the cell's progression through the stages of the cell cycle and are, in turn, regulated by numerous proteins, including p53, p21, p16, and cdc25. Downstream targets of cyclin-cdk complexes include pRb and E2F. The cell cycle often is dysregulated in neoplasia due to alterations either in oncogenes that indirectly affect the cell cycle, or in tumor suppressor genes or oncogenes that directly impact cell cycle regulation, such as pRb, p53, p16, cyclin D1, or mdm-2 (for review see Schafer, 1998).
P21, also known as CDNK1A (cyclin-dependent kinase inhibitor 1A), or CIP1 inhibits mainly the activity of cyclin CDK2 or CDK4 complexes. Therefore, p21 primarily blocks cell cycle progression at the G1 stage of the cell cycle. The expression of p21 is tightly controlled by the tumor suppressor protein p53, through which this protein mediates the cell cycle G1 phase arrest in response to a variety of stress stimuli. In addition, p21 protein interacts with the DNA polymerase accessory factor PCNA (proliferating cell nuclear antigen), and plays a regulatory role in S phase DNA replication and DNA damage repair.
After DNA damage, many cells appear to enter a sustained arrest in the G2 phase of the cell cycle. Bunz et al. (1998) demonstrated that this arrest could be sustained only when p53 was present in the cell and capable of transcriptionally activating the cyclin-dependent kinase inhibitor p21. After disruption of either the p53 or the p21 gene, gamma-radiated cells progressed into mitosis and exhibited a G2 DNA content only because of a failure of cytokinesis. Thus, p53 and p21 appear to be essential for maintaining the G2 cell cycle checkpoint in human cells.
Due to the connection between the transcriptional activity of p53 and p21 RNA expression, the readout of p21 RNA can be used to determine the effect of drugs or other insults (radiation, antisense for a specific gene, dominant negative expression) on a given cell system which contains wild type p53. Specifically, if a gene is removed using antisense products and this has an effect on the p53 activity, p21 will be upregulated and can serve therefore as an indirect marker for an influence on the cell cycle regulatory pathways and induction of cell cycle arrest.
In addition to cancer regulation of cell cycle activity has a role in numerous other systems. For example, hematopoietic stem cells are relative quiescent, while after receiving the required stimulus they undergo dramatic proliferation and inexorably move toward terminal differentiation. This is partly regulated by the presence of p21. Using p21 knockout mice Cheng et al. (2000) demonstrated its critical biologic importance in protecting the stem cell compartment. In the absence of p21, hematopoietic stem cell proliferation and absolute number were increased under normal homeostatic conditions. Exposing the animals to cell cycle-specific myelotoxic injury resulted in premature death due to hematopoietic cell depletion. Further, self-renewal of primitive cells was impaired in serially transplanted bone marrow from p21 −/− mice, leading to hematopoietic failure. Therefore it was concluded that p21 is the molecular switch governing the entry of stem cells into the cell cycle, and in its absence, increased cell cycling leads to stem cell exhaustion. Under conditions of stress, restricted cell cycling is crucial to prevent premature stem cell depletion and hematopoietic death. Therefore, genes involved in the downregulation of p21 expression could have a stimulatory effect and therefore be useful for the exploration of stem cell technologies.
Using the above examples, it is clear the availability of a novel cloned leucine-rich repeat containing protein provides an opportunity for adjunct or replacement therapy, and are useful for the identification of leucine-rich repeat containing protein agonists, or stimulators (which might stimulate and/or bias leucine-rich repeat containing protein action), as well as, in the identification of leucine-rich repeat containing protein inhibitors. Hence it can be reasoned that agonists and antagonists for these LLR containing proteins will be useful for therapeutic purposes.
The present invention also relates to recombinant vectors, which include the isolated nucleic acid molecules of the present invention, and to host cells containing the recombinant vectors, as well as to methods of making such vectors and host cells, in addition to their use in the production of HLRRBM1 polypeptides or peptides using recombinant techniques. Synthetic methods for producing the polypeptides and polynucleotides of the present invention are provided. Also provided are diagnostic methods for detecting diseases, disorders, and/or conditions related to the HLRRBM1 polypeptides and polynucleotides, and therapeutic methods for treating such diseases, disorders, and/or conditions. The invention further relates to screening methods for identifying binding partners of the polypeptides.